It is my understanding that the Red Hat "Releases" are just up to date RPM's of the Linux OS. If you update your system regularly with the newest RPM's as they become released, technically you are upgrading through the Red Hat "Releases". So as you installed 7.3, and you upgraded everything as the upgrades came out, then you should be all set.

I believe Red Hat stopped supporting 7.2 and 7.3 because they are so old and that the packages/versions are so outdated that they are security risks. It just makes more sense to release something that has all the fixes in it already then to have to patch up an earlier version.

As Red Hat progresses through the versions their installers usually progress as well, which sorta gives the illusion of a whole new OS, when in fact the latest OS is available the moment a new kernel or packages comes out.

Best way to keep up to date is to sign up for the Red Hat Network. If you go to http://rhn.redhat.com and register, you can set your server up to update your profile, and you can keep your system up to date. It's sorta like windowsupdate.com. You would use up2date on your server and it would tell you what you need to update, and you can have it update automatically. But, with Red Hat dropping support for 7.3, you would probably have to update your server manually at some point. And this is where the RPM's come into play. You would just download the appropriate RPM's, and then update your system with them. You would really need to read up on this from their site, as it is a pretty big subject. I couldn't even begin to document it here. I would suggest starting at rhn.redhat.com and then start reading about RPM's in the howto's. Then get a feel for what it's like updating things, and then you should be fine in keeping your server up to date. Then make sure you install the latest version on your new servers, so you can make use of up2dates automatic'ness.

If you are planning to or are running Chilisoft ASP (now Sun One Active Server Pages), you should note that it is not compatible with Red Hat 8+. The glibc version required by 8+ is the problem, so beware. Sun has not commented upon when they will have an updated release that supports the newer glibc versions.